1. Field of the Invention
The present invention relates to a lens grinding method and a lens grinding apparatus for grinding a spectacle lens by adjusting a distance between a lens rotation shaft for sandwiching and rotating a spectacle lens and a grindstone rotation shaft for grinding a spectacle lens into a lens shape such as a spectacle frame and in particular, to control of rotation speed of the lens rotation shaft for rotating the spectacle lens.
2. Description of the Prior Art
In a conventional lens grinding apparatus, a carriage is mounted on the apparatus main body in such a manner that the carriage can rotate around a rear end portion to swing upward end downward, a pair of lens rotation shafts arranged at right and left on a single axis are rotatably held at shaft mounting protrusions at right and left of the carriage. One of the lens rotation shafts can be adjusted so as to proceed and recess with respect to the other lens rotation shaft. Rotation drive means is provided for the lens rotation shafts and raising/lowering means is provided for swinging the lens rotation shaft and the carriage upward and downward A grindstone is rotatably held on the apparatus at a position below a lens arranged between the pair of lens rotation shads so as to be processes A calculation control circuit is provided for driving/controlling the rotation drive means and the raising/lowering means according to spectacle lens shape information (xcfx81n, nxcex8).
This spectacle lens shape information (xcfx81n, nxcex8) may include lens frame shape of a spectacle frame lens and lens model of a rimless frame. This spectacle lens shape information is normally measured by a lens frame shape measuring apparatus such as a frame reader and is transferred to a lens grinder. It should be noted that the spectacle lens shape is not a strict circle but a complicated shape consisting of a curved portion and a straight portion or an indented arc portion. The calculation control circuit of the lens grinder drives and controls the rotation drive means so as to rotate/drive the lens rotation shaft, thereby rotating the lens held on the lens rotation shaft so as to be processed. On the other hand, the calculation control circuit operate the raising/lowering means based on the spectacle lens shape information (xcfx81n, nxcex8), thereby raising/lowering the carriage. With this control, a periphery of the lens to be processed is ground into a spectacle shape by the grindstone.
Here, as shown in FIG. 13A, the lowest position of the lens rotation shaft by the self-weight of the carriage is adjusted for each rotation angle nxcex94xcex8 by the raising/lowing means, thereby adjusting a distance Ln between the lens rotation shaft line O1 at the rotation angle nxcex94xcex8 and the rotation center (rotation shaft line) O2 of the grindstone Q, so that the lens LE to be processed is ground into a spectacle lens shape.
In this grinding process, at the maximum radius value xcfx81max of the spectacle lens shape information (xcfx81n, nxcex8), the lens LE to be processed is in contact with the grindstone Q on a virtual straight line S connecting the lens rotation center O1 and the rotation center O2 of the grindstone Q.
However, as grinding of the periphery of the lens to be processed proceeds, the lens LE is scarcely brought into contact with the grindstone Q on the aforementioned virtual straight line S. Especially when finishing grinding (polishing) is performed by the finishing grindstone (grindstone Q), the periphery of the lens LE is already in an approximately spectacle lens shape and accordingly, as shown in FIG. 13B, the straight line portion La and the indented arc portion (not depicted) of the lens LE are brought into contact with the grindstone Q at a position P on the virtual straight line S only via their intermediate position and the other portions are brought into contact at a position P shifted in the circumferential direction from the virtual straight line S.
That is, in case of an acute angle portion Lb of the periphery of the lens LE, change of fine rotation angle of the lens to be processed does not change significantly the shift amount of the contact position of the periphery of the lens LE with the grindstone Q. However, at the straight line portion La and the indented portion, sight rotation of the lens LE causes a great shift amount of the contact position of the periphery of the lens LE with the grindstone Q.
Accordingly, when a lens is rotated at a constant speed as in the prior art, the contact time between the grindstone Q and the lens LE differs according to the spectacle shape. That is, the contact time between the acute angle portion Lb and the grindstone Q becomes long while the contact time at the straight line portion La becomes short Accordingly in the conventional grinding method, even when an accurate spectacle lens shape information (xcfx81n, nxcex8) is obtained, it has been impossible to obtain an accurate spectacle lens shape based on this spectacle lens shape information (xcfx81n, nxcex8) because of the difference in the grinding condition (state) at the periphery
That is, in case the lens shape data is circular, even when a lens to be processed has become almost a lens shape by grinding, the lens is still circular. For this, when the lens to be ground by the grindstone is lowered, the lens and the grindstone are in contact with each other at a constant speed at any portions of the lens and the grindstone. However, when the lens shape information is rectangular, as the lens to be processed approaches a lens shape, the lens becomes more and more rectangular. In the lens which has become almost rectangular; when_an apex defined by two sides of the rectangular shape is brought into contact with the grindstone, the contact time on the grindstone becomes longer. On the other hand, when a center portion of each of the sides of the rectangular lens shape is considered, the entire side of the lens to be ground according to the lens shape data is in contact with the periphery of the grindstone while rotating along the grindstone. Thus the lens is in contact with only one point on the grindstone and the contact time becomes very short
To work around this, Japanese Patent Laid-open No. Hei 09-277148 discloses a lens grinding method and a lens grinding apparatus in which a contact time of a lens to be processed, with a grindstone is adjusted according to a spectacle shape by considering a shift amount of the contact position between the lens and the grindstone in a peripheral direction, thereby enabling to accurately grind a spectacle lens shape.
In this lens grinding apparatus, according to the lens shape data (xcfx81n, nxcex8) for processing a lens periphery measured by lens shape measuring means, a lens to be processed is rotated and simultaneously with this, advanced/recessed for each rotation angle nxcex8 while the periphery of the lens is ground by the grindstone into a spectacle lens shape. Moreover, using the lens shape data (xcfx81n, nxcex8) and the radius of curvature of the grindstone, calculation is performed to obtain a difference angle dxcex8n defined by a virtual processing point at radius vector xcfx81n of the rotation angle nxcex8[n=0, 1, 2, 3, . . . j] and an actual contact processing point of the lens with the grindstone at rotation angle nxcex8, and the rotation angular speed of the lens is controlled according to the angle dxcex8n at this rotation angle nxcex8, so that the contact time of the grindstone at the rotation angle nxcex8 is approximately constant.
In this lens grinding apparatus, correction data based on the obtained angle dxcex8n is called from a correction table. This correction data and a reference rotation speed of the lens rotation shaft are used to calculate a correct a lens rotation speed to be corrected, which in turn is used for controlling the rotation speed of the lens rotation shaft (lens to be processed).
However, conventionally, it is necessary to store correction data based on the angle dxcex8n for each of the lens materials in the correction table. Nowadays, various lens materials are used and it is difficult to obtain correction data for each of the lens materials to prepare a correction table.
In this correction method, dxcex8n differs according to the frame shape data and accordingly, a rant requiring rotation correction and the correction amount are not constant, causing a problem that a significant time is required.
When considering a frame shape as a limited number of radius vector data items, a radius vector of the lens points to a center of the grindstone. To indicate this direction, an angle with respect to a rotation reference position when the lens shape is represented in spherical coordinates is called a rotation angle, and an angle dewed by a radius vector of the rotation angle when the lens shape in contact with the grindstone represented in spherical coordinates and a radius vector of the contact point of the lens with the grindstone is called a contact angle. The total of the rotation angle and the contact angle will be referred to as a processing angle. Here, consideration is taken for a case when an apex of an approximately rectangular lens is in contact and a case when a side of the approximately rectangular lens is in contact. When the apex is in contact, the contact angle does not change while the rotation angle greatly changes. As a result, as the rotation angle changes, the processing angle greatly changes its sign from positive to negative together with a great change of its absolute value. In contrast to this, when the side is in contact, a change of the contact angle is greater than a change of the rotation angle. It looks like that the contact angle, sandwiching the middle point of the side of the rectangular lens, outruns the processing angle. In the same way as this, the processing angle becomes 0 at the middle point of the side, changes its s from negative to positive, and its absolute value is also greatly changed.
Thus, in the conventional correction time based on the processing angle, as the processing angle increases, its time is also increased. However, as has been described above, at the different processing conditions at the apex and the side, the processing angle changes its sign passing 0. That is, at the apex and at the middle point of the side where the speed should be different, actually the velocities coincide. Accordingly, it has been impossible to obtain a constant contact time at different points.
It is therefore an object of the present invention to provide a lens grinding method and an apparatus for the same capable of giving a rotation speed stabilizing the contact time at different points and changing the stability of the rotation speed at different points according to the material and the processing conditions.
In order to achieve the aforementioned object, the lens grinding method according to the present invention changes a lens rotation speed when grinding a spectacle lens by using spectacle frame lens shape information, spectacle lens information such as spectacle lens material, and various spectacle processing information required for processing a spectacle lens.
According to another aspect of the present invention, a predetermined angle for the rotation reference position when the spectacle frame lens shape is expressed in spherical coordinates is referred to as a rotation angle while an angle for the rotation reference position when the lens shape in contact with the grindstone is expressed in spherical coordinates is referred to as a contact angle and calculation is performed to obtain a rotation angle change of the lens rotation shaft holding the spectacle lens per a predetermined time as a rotation angular speed and a contact angle change per a predetermined time as a contact angular speed, so that the rotation speed of the lens rotation shaft is controlled by the speed obtained by combining the speed component in which the rotation angle rotates at a constant speed and the speed component in which the contact angle rotates at a constant speed.
According to yet another aspect of the present invention, a predetermined angle for the rotation reference position when the spectacle frame lens shape is expressed in spherical coordinates is referred to as a rotation angle while an angle for the rotation reference position when the lens shape in contact with the grindstone is expressed in spherical coordinates is referred to as a contact angle and calculation is performed to obtain a rotation angle change of the lens rotation shaft holding the spectacle lens per a predetermined time as a rotation angular speed and a contact angle change per a predetermined time as a contact angular speed, so that the rotation speed of the lens rotation shaft can change with an arbitrary combination ratio of the speed component in which the rotation angle rotates at a constant speed and the speed component in which the contact angle rotates at a constant speed.
According to still another aspect of the present invention, there is provided a lens grinding apparatus comprising: a lens rotation shaft for sandwiching and rotating a spectacle lens; drive means for driving the lens rotation shaft; a grindstone for grinding a spectacle lens; shaft-to-shaft distance adjusting means for controlling a distance between a lens rotation shaft and a grindstone; and calculation control means which operates in such a manner that a predetermined angle for the rotation reference position when the spectacle frame lens shape is expressed in spherical coordinates is referred to as a rotation angle while an angle for the rotation reference position when the lens shape in contact with the grindstone is expressed in spherical coordinates is referred to as a contact angle and calculation is performed to obtain a rotation angle change of the lens rotation shaft holding the spectacle lens per a predetermined time as a rotation angular speed and a contact angle change per a predetermined time as a contact angular speed, so that the rotation speed of the lens rotation shaft is controlled by the speed obtained by combining the speed component in which the rotation angle rotates at a constant speed and the speed component in which the contact angle rotates at a constant speed.
According to yet still another aspect of the present invention, the lens grinding apparatus comprises: a lens rotation shaft for sandwiching and rotating a spectacle lens; drive means for driving the lens rotation shaft; a grindstone for grinding a spectacle lens; shaft-to-shaft distance adjusting means for controlling a distance between a lens rotation shaft and a grindstone; and calculation control means which operates in such a manner that a predetermined angle for the rotation reference position when the spectacle frame lens shape is expressed in spherical coordinates is referred to as a rotation angle while an angle for the rotation reference position when the lens shape in contact with the grindstone is expressed in spherical coordinates is referred to as a contact angle and calculation is performed to obtain a rotation angle change of the lens rotation shaft holding the spectacle lens per a predetermined time as a rotation angular speed and a contact angle change per a predetermined time as a contact angular speed, so that the rotation speed of the lens rotation shaft can change with an arbitrary combination ratio of the speed component in which the rotation angle rotates at a constant speed and the speed component in which the contact angle rotates at a constant speed.